A high-efficiency condensing furnace is a gas furnace with a secondary heat exchanger that cools exhaust gases until their water vapour condenses, recovering extra heat. That pushes efficiency to roughly 90–98% AFUE. Because the exhaust is cool and acidic, it vents through plastic pipe and produces condensate that must drain away.
Why it matters to you
Condensing furnaces are the standard choice for new gas installs in BC and the backup half of many dual-fuel systems. They run wet by design, so in Fraser Valley homes a plugged condensate line or a frozen exterior vent is a common cause of mid-winter no-heat calls.
Practical guidance
When replacing an older furnace, ask how the new unit will vent, where the condensate will drain, and whether a condensate pump is needed. Keep the outdoor intake and exhaust pipes clear of snow, leaves, and shrubs.
Why does a condensing furnace need a drain?
Pulling extra heat out of the exhaust cools it enough for water vapour to condense into liquid. That condensate is mildly acidic and has to go to a floor drain or a condensate pump. If the line plugs, a safety switch stops the burner rather than let water back up — a common no-heat cause on high-efficiency units.
Can my old chimney be reused with a condensing furnace?
Generally no. Condensing furnaces vent through sealed plastic pipe, usually out a sidewall or the roof, not through a masonry chimney. If a gas water heater still shares that chimney, removing the furnace can leave the chimney oversized for the water heater alone, which a gas fitter needs to assess.
Is upgrading from a mid-efficiency furnace worth it?
In gas terms, an 80% furnace replaced by a 96% condensing model uses about 17% less gas for the same heat. Whether that justifies replacement now depends on the old furnace's age and condition, your ductwork, and whether a heat pump or dual-fuel setup suits the home better. An honest assessment looks at all of those.